Operability, Resiliency, and Flexibility: process design objectives for a changing world

Chemical plants have to be designed based on uncertain thermodynamic, kinetic and equipment performance correlations, market forecasts, raw material and product prices. They have to be able to adjust to changes in operating conditions, disturbance levels, product specifications, product distribution and demand, and they should be able to tolerate equipment malfunction without leading to serious safety hazards. The general term "operability" will be used to describe the ability of the plant to perform satisfactorily under conditions different from the nominal design conditions. Current industrial practice accounts for operability at the design stage in an ad hoc fashion through empirical overdesign factors and by introducing large storage and surge tanks for raw materials, products and intermediates. It is shown here that the heuristic approach is not only often costly and ineffective but that it can have an adverse